120V and 220V AC in the same control cabinet

Short answer: land one 240 V feed on the cabinet, run the servo drives straight off it, and step down to 120 V with an industrial control transformer inside. Two 200 W servo axes, a 12 A 24 VDC supply and the control loads draw 4.7 A at the high service against 9.4 A at the low one — half the current, one size smaller lugs, one lockout point. Nothing in the NEC or UL 508A stops the two voltages from sharing one enclosure. The rules govern something else: one disconnect per feed, a marking that names each source, an interlocked door wherever you can touch conductors above 50 V, and components built for panel use. The 2000 W converter from the home-improvement aisle fails that last test. A 1000 VA control transformer passes all of it, and the wiring below shows why it also saves you copper.

Why one feed beats two inputs

Power is volts times amps, so the same cabinet at half the feed voltage pulls double the current. The saving lands where panels feel it: conductors, breaker frames, disconnect ratings and lug space. The table is single-phase arithmetic, amps = VA ÷ volts, so you can scale your own load in one move.

Worked to wire size, 4,000 VA × 1.25 for continuous load = 5,000 VA. At the low voltage that is 41.7 A, which pushes you to 8 AWG copper, rated 50 A at 75 °C in the NEC ampacity table for copper conductors at 75 °C. The same 4 kVA at the high one is 20.8 A after the multiplier and runs on 12 AWG, rated 25 A. That is why step-up builds cluster where only the lower service exists — booths and halls with no higher service in the wall — rather than in plants that have it.

One point about the space in front of the cabinet: a 240/120 V single-phase feed sits 120 V to ground on each leg, so the 3 ft (900 mm) working-space depth of NEC 110.26(A) covers it.

What you need

• 240 V single-phase, 3-wire feed: two hots plus ground. Add a neutral only when something inside must run straight off the service at the lower voltage.
• 30 A, 2-pole disconnect switch, padlockable, with the handle through the door.
• 15 A, 2-pole breaker for the feeder, right after the disconnect.
• Two servo drives with single-phase input at the higher voltage, plus the line fuses their manual names.
• 1000 VA industrial control transformer, 240 V to 120 V, with primary fusing per its marking.
• 24 VDC power supply, 12 A, fed from the transformer secondary.
• 14 AWG copper throughout the feeder; black, red, white and green insulation.
• A label kit, and the multi-source warning plate if a second feed ever lands on this cabinet.

Wiring the panel, step by step

1. Mount the disconnect and land the feed. Put the 30 A, 2-pole switch on the door; NEC 240.24(A) keeps the center of the handle grip at most 2.0 m (6 ft 7 in) above the floor, and the door mount keeps it that way as you reach for it. Size the conductors for the breaker, not the switch: 14 AWG copper with a 14 AWG equipment grounding conductor, which NEC Table 250.122 sets from the 15 A overcurrent device ahead of it.
2. Break the feeder at 15 A. The worked example above lands at 5.9 A after the 1.25 multiplier, so a 15 A, 2-pole breaker carries the whole cabinet with room for inrush. Mark the handle for its purpose, as the NEC asks of every disconnecting means: a short "panel feed" tag naming the voltage is enough.
3. Feed the drives at the service voltage. Land each drive on the black and red legs and fuse its line side with the class and rating its manual names. That fuse value belongs to the drive document; guessing it smaller to save copper is how nuisance trips start.
4. Add the control transformer. Mount the 1000 VA unit, fuse the primary per its marking, then bond the grounded secondary terminal to the ground bar after clearing paint from the contact surface (NEC 250.12). A floating secondary hides the first fault until a second one makes the cabinet live; grounding and bonding covers the details.
5. Distribute the 120 V. Run the secondary's black hot and white grounded conductor to the relays, the panel light and the 24 VDC supply in their own wire duct. Label both ends of every run so nobody reads the secondary hot as a leg of the incoming service.
6. Interlock the door. Every conductor above the 50 V shock line NFPA 70E draws sits behind a door that opens only with the disconnect handle or a tool; what each door needs for interlocking sorts door by door. The handle's door cutout then needs the bonding jumper sized for the door joint.
7. Mark the finished panel. Attach the multi-source warning marking UL 508A specifies whenever more than one feed enters, mark the short-circuit current rating — the lowest SCCR among the installed power components, as an internal transformer and the panel's SCCR explains — and carry the enclosure type number that NEC 409.110 expects on the panel marking. Then lock out and confirm absence of voltage before the first hand goes in.

When a second input earns its keep

Some sites want the two voltage worlds kept apart, and NFPA 79 and UL 508A let a panel carry both supplies legally. Each input then needs its own disconnect, its own overcurrent protection, a marking naming its source, and an interlock on every door that reaches above 50 V. Your lockout sheet names both feeds, and the habits in the mixed-voltage safety checklist start applying on day one. If the second feed is the lower voltage, when a 120 V enclosure needs its own disconnect settles that question first.

Panel shops that split voltages usually stack two cabinets instead: one gland between them, carrying the same manufacturer rating as the boxes, since type ratings on our boxes are manufacturer ratings and not UL listings. The 24 VDC cabinet stays unlocked for daily work; the one holding the higher service sits behind a lock. If the second disconnect lives in its own small enclosure, remember its line side stays live with the main breaker off.

Why the home-store converter fails

A 2000 W consumer voltage converter fails a panel build three ways. It carries no evaluation as an industrial control component, so a UL 508A shop cannot put it in a listed panel. It is listed as a portable, plug-in appliance, and hard-wiring it into a cabinet puts it outside that listing the moment the glands go in. And OSHA 29 CFR 1910.303(a) expects electrical equipment to be approved, with 1910.399 reading approved as accepted, certified, listed, labeled or otherwise determined safe by a nationally recognized testing laboratory.

Moving the converter outside the cabinet moves the hardware, not the problem. The machine's power still flows through it, it still needs an enclosure, overcurrent protection and an approval basis, and the inspector looks at the machine you build, not at one box. The fix is dull and light on panel space: a 1000 VA industrial control transformer, a part made for this duty. The logic behind parts and listings is what UL 508A listing involves, and running an unlisted enclosure in a listed panel covers the cousin case.

Size the cabinet for the transformer's heat

A 1000 VA control transformer takes about 8 x 8 in (200 x 200 mm) of backplate; leave 2 in (50 mm) above its terminals for the primary and secondary landing space. Then it heats the box. Dry-type transformer losses run about 1-3% of rating, so take the top of that band: 30 W from the transformer, roughly 15 W from each drive inside, 10 W from the 24 VDC supply — about 70 W of internal loss.

Stainless walls pass heat at about 3.7 W/m²K, so a 20 x 24 x 10 in (508 x 610 x 254 mm) box with 1.19 m² of surface rises 70 ÷ (3.7 × 1.19) ≈ 16 K above ambient. At a 25 °C shop floor that is 41 °C at the walls, already over the 40 °C ambient that drives and most electronics carry unless a datasheet allows more, and 51 °C on a 35 °C afternoon. Two moves fix it: step up a box size so the walls carry more area, or plan sealed cooling for the transformer's corner. The full walk-through is in the enclosure sizing guide, and NEC 312.8 caps how much of a cabinet holding overcurrent devices may fill with conductors and splices, so keep spare space on the backplate.

Checks before you close the door

• Measure the secondary at no load: a few volts above 120 V is normal transformer behavior; loaded, the bus sits within 10% of nominal and the 24 VDC rail within 5%.
• Confirm continuity from the secondary's grounded terminal to the ground bar — tenths of an ohm, the same reading you get on the main ground path.
• Fill every unused breaker space with a filler plate and fit the dead front before the door swings.
• Read the labels from 3 ft away: source marking, multi-source plate, SCCR, enclosure type number.
• After 30 minutes at full load, put a thermal camera over the lugs and the transformer; any termination more than 10 K above its neighbor gets retorqued.

The decision, by what the building gives you

Feed high and transform down, and your machine has one lockout point and one story on the label. The two-input build is legal; it multiplies the marking, the interlocks and the chances that someone misses a live leg. Whatever you pick, the box itself comes from custom stainless enclosures cut to your dimensions, with the disconnect operator and gland cutouts laid out in the custom enclosure builder.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions